Conventionally, there have been proposed various types of seatbelt retractors including a pretensioner that removes slack of a webbing to protect a vehicle occupant at the time of emergency such as vehicle collision and the like.
For instance, Japanese Patent No. 3519302 discloses a pretensioner for a seatbelt retractor which intends cost reduction in assembly and processing thereof. The pretensioner disclosed in Japanese Patent No. 3519302 includes a piston which moves within a cylinder with the aid of gas pressure generated at the time of vehicle emergency so as to rotate a webbing-take-up axis in a webbing-take-up direction. Thereafter, a ring-like sealing member is fitted to the periphery of a seal holding member integrally formed on a gas-pressure-receiving-side surface of the piston so as to seal a space between the piston and an inner wall of the cylinder.
Further, in recent years, pretensioner mechanisms for seatbelt retractors are generally combined with energy absorption mechanisms when used. An energy absorption mechanism pulls out a webbing for relaxing impact on a vehicle occupant in case tensile force on the webbing exceeds a predetermined value after slack of a webbing is removed by a pretensioner mechanism. An energy absorption mechanism pulls out a webbing while reducing load so that a predetermined value or more of load should not be applied to a vehicle occupant. For the purpose of reducing load when a webbing is pulled out, for instance, the energy absorption mechanism allows a take-up drum to rotate in a webbing-pull-out direction while efficiently applying pull-out-resistance to the take-up drum.
In this case, gas pressure remaining in the cylinder of the pretensioner mechanism affects webbing-pull-out operation at an energy absorption mechanism. Japanese Patent No. 3218987 teaches to include a gas releasing mechanism so as to prevent remaining gas from disturbing operation to be done by an energy absorption mechanism.
In case of assembling the pretensioner mechanism disclosed in Japanese Patent No. 3519302, the sealing member is fitted to the periphery of the seal holding member and the sealing member and the piston are inserted in the cylinder. However, the sealing member with a ring-like shape may raise a fear to get twisted, deviated or displaced due to friction against the inner wall of the cylinder. As a result, there may be caused deterioration of sealing function due to a space between the piston and the inner wall of the cylinder and leakage of gas to press and drive the piston, which is problematic.
In case of the configuration disclosed in Japanese Patent No. 3218987, gas leaks from a gas releasing hole provided on the gas releasing mechanism at an initial activation stage of the pretensioner mechanism. If diameter of the gas releasing hole is made large enough to allow activation of the energy absorption mechanism comfortably, gas leaks considerably. As a result, the pretensioner mechanism cannot carry out webbing slack removal operation efficiently due to the considerable gas leakage. If diameter of the gas releasing hole is contrarily made small for the pretensioner mechanism to efficiently carry out webbing slack removal operation, operation of the energy absorption mechanism may be disturbed.